Mandukya Karika, verse 1.15


अन्यथा गृह्णतः स्वप्नो निद्रा तत्त्वमजानतः ।
विपर्यासे तयोः क्षीणे तुरीयं पदमश्नुते ॥ १५ ॥

anyathā gṛhṇataḥ svapno nidrā tattvamajānataḥ |
viparyāse tayoḥ kṣīṇe turīyaṃ padamaśnute || 15 ||

15. Svapna or dream is the wrong cognition of Reality. Nidrā or sleep is the state in which one does not know what Reality is. When the erroneous knowledge in these two disappears, Turīya is realized.

Shankara Bhashya (commentary)

When is one established in Turīya? It is thus replied: During the states of dream and waking when one wrongly cognizes Reality like the perception of the snake in the place of the rope, he is said to be experiencing dream.1 Nidrā or sleep,2 characterised by the ignorance of Reality, is the common feature of the three states. Viśva and Taijasa, on account of their having the common features of Svapna (dream) and Nidrā (sleep), form a single class. That Nidrā (sleep) which is characterised by the predominance of wrong apprehension (of Reality) constitutes the state of inversion which is Svapna (dream). But in the third state, Nidrā (sleep), alone, characterised by the nonapprehension of Reality is the only inversion. (This forms the second or the other class implied in the text which speaks only of dream and sleep as covering the three states.) Therefore when these two classes of the nature of effect and cause, characterised by the mis-apprehension and non-apprehension respectively (of Reality), disappear by the destruction of the inversion characterised by effect and cause, by the knowledge of the nature of the Highest Reality, then one realises Turīya which is the goal. Then one does not find in Turīya this condition, the characteristics of which are these two (effect and cause), and one thus becomes firm in the Highest Reality which is Turīya.